The present invention relates to a projection exposure method for exposing a pattern as an original print, such as a mask or reticle (hereinafter referred to collectively as a xe2x80x9cmaskxe2x80x9d), to a photosensitive substrate, such as a wafer, in a photolithography process for manufacturing semiconductor elements, such as LSIs, image pickup elements, CCDs, liquid crystal display elements or thin film magnetic heads, to an projection exposure apparatus for use in the projection exposure method, and to methods for manufacturing and optically cleaning the exposure apparatus.
As a degree of integration of semiconductor elements becomes higher, rapid progress has also been made of projection exposure apparatuses for use in a photolithography process that is important for the manufacture of such semiconductors. The resolving power of a projection optical system to be loaded on a projection exposure apparatus may be represented, as it is well known as Rayleigh""s formula, by the relationship as represented by R=kxc3x97xcexxc3x97NA, wherein R is the resolving power of a projection optical system, xcex is the wavelength of a light for use in exposure, NA is an aperture number of the projection optical system, and k is a constant to be determined by the process, in addition to the resolving power of a photoresist.
In order to realize the resolving power required for the projection optical system in correspondence with a high degree of integration of semiconductor elements, efforts have been continued to shorten the exposure light source or to enlarge the number of apertures of the projection optical system, i.e. to make the NA higher. Recent years, an exposure apparatus using krypton fluoride excimer laser beam (KrF excimer laser beam) with an output wavelength of 248 nm as an exposure light beam source and having the aperture number of 0.6 or larger of the projection optical system can be commercially available so that the exposure to a pattern as fine as 0.25 xcexcm can be effected.
Further, an argon fluoride excimer laser (ArF excimer laser) with an output wavelength of 193 nm has recently been put to practical use after the krypton fluoride excimer laser. If an exposure apparatus could be realized which uses the argon fluoride excimer laser beam as an exposure light beam source, it can be expected that the processing as fine as 0.8 xcexcm to 0.13 xcexcm can be effected. Great efforts have currently been continued to make research to develop such an exposure apparatus that can use the such laser beam.
The material for use as a lens is currently restricted to only two materials, i.e., synthetic quartz glass and calcium fluoride (fluorite) glass, from the point of view of transmittance or the like in the wavelength region of the output wavelength (193 nm) of the argon fluoride excimer laser. Therefore, extensive efforts have been continuously made to develop optical material for use with an exposure apparatus of this kind, which has a sufficient level of transmittance and an inner material uniformity. Synthetic quartz glass having an inner transmittance of 0.995/cm or higher and calcium fluoride having a negligible level of inner absorption have currently been developed.
Material to be coated on the surface of an optical material for use in preventing reflection is very narrow in a selection scope and undergoes great restrictions upon the freedom of design, as compared with material in a wavelength region of the output wavelength (248 nm) of krypton fluoride excimer laser. However, the problems are being overcome by extensive development efforts, and the loss at each lens plane is being realized to a level below 0.005 or less.
In a wavelength region shorter than the wavelength of the KrF excimer laser beam, the problems may be caused to arise such that the transmittance or reflectance of an optical system is caused to be reduced (fluctuate) due to the phenomenon that if water or organic material would be attached to the surfaces of optical elements constituting the optical system (illumination optical system or projection optical system) in a projection exposure apparatus. In other words, the problems with a variation or fluctuation in an attenuation factor of the optical system may be caused to arise due to a variation or fluctuation in the transmittance or reflectance. The such problems may result from the attachment of impurities to the surface of the optical system, such impurities including water, hydrocarbons or organic substances for example, derived from gases present in a space interposed between plural optical elements, the inner wall of a lens barrel supporting the optical system, or adhesive.
FIG. 14 shows features of a periodical variation in transmittance of an optical system. The periodical variation is represented by measuring an illuminance of an exposure light beam on a portion between a laser beam source and a mask and an illuminance of the exposed light on a wafer at predetermined time intervals while emitting pulse laser beams continuously from the laser beam source and calculating a ratio of the illuminance of the exposure light beam to the illuminance of the exposed light beam at every measured time as a variation in transmittance of the optical system. As it is apparent from FIG. 14, the transmittance is caused to be reduced to a great extent immediately after the start of irradiation with a laser beam and thereafter the transmittance arises gradually to reach an almost saturated state as some time elapses. The reduction in transmittance immediately after the start of the irradiation with laser beam is caused by a fluctuation in the inner features of a glass material, and the transmittance is gradually recovered thereafter due to the fact that water or organic substances attached to the surface of the optical system is or are removed by the irradiation with laser beam.
Therefore, the transmittance of the illumination optical system or the projection optical system is caused to arise gradually during the exposure operation of the projection exposure apparatus, that is, upon the transfer of an image of a device pattern formed on a mask onto a photosensitive substrate in a step-and-repeat manner or a step-and-scan manner by illuminating the mask with an exposure light beam from the illumination optical system and projecting at least a part of the image of the device pattern formed on the mask onto the photosensitive substrate by means of the projection optical system. It can be noted herein that the rise of the transmittance represents the effect of temporarily cleaning an projection exposure apparatus, and the optical system with the surface of the optical element activated by the irradiation with an exposure light beam may become rather likely to attract water or organic material present in the surrounding once the irradiation with the light beam would be suspended. With the such phenomenon kept in mind, in the case where the irradiation with the exposure light beam (the exposure operation) is suspended for a long time or for a long period of time, it is considered that the transmittance is made nearly saturated as needed by effecting the optical cleaning by the irradiation with the exposure laser beam for a predetermined period of time prior to the start of exposure and thereafter the exposure operation has been started. This operation is undesirable, however, because it may cause decreasing throughput. Further, the oscillation of laser beam for a long period of time prior to exposure is likewise undesirable because it may result in a decrease in durability of a laser beam source, in addition to a reduction in throughput. Moreover, it is difficult to continue the irradiation with the exposure laser beam all the time, including the time for exchanging wafers and masks.
Then, a description will be made of the problems that may result from the fluctuation in the transmittance of the projection exposure apparatus adapted so as to be used by shifting an optical element as a part of the illumination optical system or the projection optical system.
As the NA of the projection optical system becomes higher, the depth of focus (DOF) as a margin upon the manufacture of a pattern is caused to be reduced, the DOS being as represented by the formula as follows:
DOF=K2xc3x97xcex/NA2
where
K2 is the resolving power of a photoresist; and
xcex is a constant to be determined by the process. There is known the method for improving the resolving power or the depth of focus by changing illumination conditions (i.e., the shape or size of a secondary light source) for illuminating a mask, such as a modified illumination, including a zonal illumination, a special inclined illumination or the like, or a small a value.
This method can be used for exchanging a state of distributing the exposure light beam on a pupil plane of the projection optical system and it also uses a pupil filter for changing the state in which the light deviates on the pupil plane of the projection optical system in a similar technique. The method by which the resolving power or the depth of focus is improved can be chosen in accordance with the shape or line width of a pattern of a device to be exposed.
In the cases where a system for changing the illumination conditions is adopted, a plurality of aperture stops in various shapes for use with a zonal illumination, a modified illumination, or a small a value are formed on a circular plate called a turret plate, and either one of the aperture stops is inserted into a light path of the illumination optical system by rotating the turret plate. An optical part such as a fly-eye lens or an input lens for example can be exchanged in accordance with the selected aperture stop. Further, in the cases where an isolated pattern such as a contact hole for example is transferred on a photosensitive substrate, the pupil filter is inserted into the light path of the projection optical system. Therefore, a variety of patterns can be transferred by means of only one projection exposure apparatus onto a photosensitive substrate under optical exposure conditions such as illuminating conditions, NA or the like suitable for each pattern by using an optical system optimized for a device pattern, for example, by changing the illuminating conditions and inserting the pupil filter into a light path in accordance with kinds (e.g., shape, line width, pitch, and so on) of the device patterns to be transferred onto the photosensitive substrate.
Further, in a device on which a line-and-space pattern and an isolate pattern are combined together, reviews have been made to conduct the exposure by using a mask for use with the line-and-space pattern and a mask for use with the isolate pattern in a state in which the mask for use with the line-and-space pattern and the mask for use with the isolate pattern are superimposed on a photosensitive substrate under optimal conditions, because the conditions for optimized illumination or for the NA vary with each pattern. In this case, the features of a resist material on the substrate vary as the interval of the time between the exposure of the line-and-space pattern and the exposure of the isolate pattern becomes longer, so that the masks are exchanged or the conditions of exposure are changed in a sheet unit of a substrate or in a lot unit of substrates.
In the optical system in which a part of the optical elements is exchanged in order to change the conditions of exposure in accordance with the such device pattern, the transmittance of the optical system to be disposed in the light path therefrom can be maintained at a high level by the cleaning effect. A contamination of a part of the optical elements (optical system) which is staying standby at the retraction position, however, advances as the time elapses, due to the attachment of foreign materials such as water or organic material to the optical elements. In particular, as the conditions for exposure are changed, the optical system evacuated from the light path is activated at its surface, so that foreign materials become likely to be attached to the surface thereof. This may cause a rapid reduction in the transmittance. In the cases where the optical system staying in the standby position is used by re-insertion into the light path, the illuminance (intensity) of the exposure light beam on the photosensitive substrate is reduced as the transmittance of the optical system is reduced. This causes the exposure time at every shot to become longer so that the time for processing a sheet of the photosensitive substrate becomes longer to a considerable extent and the throughput is caused to be reduced.
Generally, the projection exposure apparatus can branch off a portion of a light flux leaving from the fly-eye lens (a secondary light source) in the illumination optical system and lead the branched light flux to a light quantity sensor (integrator sensor). The output from the light quantity sensor controls the light quantity of the laser beam exposed on the photosensitive substrate at every shot. If the transmittance of the optical system (including the projection optical system) behind a beam splitter thereof would vary to a great extent, an integrated light quantity of the exposed light reaching to the photosensitive substrate cannot be controlled accurately even if the output from the light quantity sensor would be used. For instance, if a pupil filter staying standby outside the projection optical system would be inserted into the light path, the exposure dose cannot be controlled in an accurate manner. It can thus be considered that the optical cleaning be effected by the irradiation with the exposure light beam prior to the exposure. The optical cleaning before the exposure, however, may cause the problem that may result in the reduction in the throughput for example, in the manner as have been described above. In particular, in the cases where a so-called duplicate exposure is to be effected which can form a one pattern by transferring a plurality of patterns each onto a photosensitive substrate in different exposure conditions, the exposure conditions have to be changed frequently for every pattern. Therefore, if the optical cleaning would be effected whenever the exposure condition is changed, it may cause the problem that the throughput is reduced to an extreme extent.
Therefore, the primary object of the present invention is to provide a projection exposure method, a projection exposure apparatus for use with the projection exposure method, and a method for manufacturing the projection exposure apparatus, so adapted as to hold a transmittance of an optical element staying in a retraction position at a predetermined value in order to avoid a reduction of illuminance on a photosensitive substrate upon shifting the optical element to an operation position.
The second object of the present invention is to provide a projection exposure method and an apparatus for use with the projection exposure apparatus, which can prevent a fluctuation in the transmittance of the optical system due to the exchange (shift) of a part of the illumination optical system or the projection optical system.
The third object of the present invention is to provide a projection exposure method and an apparatus for use with the projection exposure method, which can always control an integrated light quantity of the exposure light beam (exposure dose) on a photosensitive substrate, without causing a reduction in throughput even if a part of the illumination optical system or the projection optical system would be exchanged.
The fourth object of the present invention is to provide a projection exposure method and an apparatus for use with the projection exposure method, which can always maintain a higher transmittance of the illumination optical system or the projection optical system in which a part of the optical element is exchanged as the exposing conditions for exposing a photosensitive substrate are changed.
The fifth object of the present invention is to provide a projection exposure method, a projection exposure apparatus, a method of manufacturing the projection exposure apparatus, and a method of optically cleaning a projection optical system, so adapted as to control a fluctuation in transmittance or reflectance (a fluctuation in an attenuation factor of an optical system) by utilizing the self-cleansing effect of the optical system.
The exposure method according to the present invention can achieve the above objects by irradiating an optical element staying in a retraction position with a cleaning light beam in the exposure method for use with the exposure apparatus for irradiating a mask with the exposure light beam through an optical system which has a part of optical elements shifted between the operation position and the retraction position.
In the exposure method according to the present invention, the exposure light beam for illuminating the mask may be used for the cleaning light beam. As the cleaning light beam, there may further be used a light beam emitting from a cleaning light beam source disposed separately from a light beam source of the exposure light beam for illuminating the mask. The exposure method may involve the irradiation of an optical element shifted at the retraction position with the cleaning light beam during the transfer of an image of the pattern onto the substrate, or the irradiation of an optical element at the retraction position with the cleaning light beam when the substrate is not irradiated with the exposure light beam during the step of transferring the image of the pattern onto the substrate.
An alternative exposure method according to the present invention can achieve the above-mentioned objects of the exposure method for use with a projection exposure apparatus having an illumination optical system for illuminating an original print with a predetermined pattern formed thereon with the exposure light beam left from the exposure light beam source and a projection optical system for projecting the pattern on the original print illuminated by the illumination optical system onto a photosensitive substrate, wherein an optical element shifted at the retraction position is irradiated with the cleaning light beam during the predetermined pattern formed on the original print is being projected onto the photosensitive substrate.
Another alternative exposure method according to the present invention can achieve the above-mentioned objects of the exposure method for use with a projection exposure apparatus having an illumination optical system for illuminating an original print with a predetermined pattern formed thereon with the exposure light beam left from the exposure light beam source and a projection optical system for projecting the pattern of the original print illuminated by means of the illumination optical system onto the photosensitive substrate, wherein the exposure light beam emitted from the exposure light beam source is controlled so as to irradiate an optical element shifted to the retraction position and not to irradiate the photosensitive substrate when the photosensitive substrate is not irradiated with the exposure light beam in the step of the exposure process for projecting the pattern onto the photosensitive substrate, and the exposure light beam emitted from the exposure light beam source is controlled so as not to irradiate an optical element shifted to the retraction when the photosensitive substrate is irradiated with the exposure light beam in the step of the exposure process for projecting the pattern onto the photosensitive substrate.
The exposure apparatus according to the present invention can achieve the above-mentioned objects in an exposure apparatus which is arranged such that a mask is irradiated with an exposure light beam through an optical system with a part of optical elements thereof arranged so as to be shifted between an operation position and an retraction position and an image of a pattern formed on the mask is transferred onto a substrate, wherein an cleaning optical system for irradiating an optical element shifted to its retraction position with a cleaning light beam is provided.
In the exposure apparatus according to the present invention, the exposure light beam for illuminating the mask may be used for the cleaning light beam. As the cleaning light beam, there may further be used a light beam emitting from a cleaning light beam source disposed separately from a light beam source of the exposure light beam for illuminating the mask. The cleaning optical system may be provided with a branched optical element for leading the exposure light beam as the cleaning light beam, which enters from the light beam source for the exposure light beam into the mask, or with a branched optical element for leading the exposure light beam as the cleaning light beam, which enters into projection optical system for projecting an image of a pattern formed on the mask onto the substrate.
An alternative exposure apparatus according to the present invention can achieve the objects in a projection exposure apparatus having an illumination optical system for illuminating an original print with a predetermined pattern formed thereon with an exposure light beam left from an exposure light beam source and a projection optical system for projecting the pattern of the original print illuminated by means of the illumination optical system onto a photosensitive substrate, in which an optical element constituting a part of the optical system is arranged so as to be shifted between its operation position and its retraction position, which comprises a cleaning optical system for irradiating an optical element shifted to its retraction position with a cleaning light beam during the projection of the predetermined pattern onto the photosensitive substrate.
In the exposure apparatus according to the present invention, the cleaning optical system is provided with a branched optical element for leading an illumination light beam as the cleaning light beam, which is emitted from the exposure light beam source. The branched optical element is arranged so as to control the exposure light beam in such a way that the exposure light beam is led to the cleaning optical system when the exposure light beam is not irradiated onto the photosensitive substrate in the exposure for projecting the pattern onto the photosensitive substrate, and that the exposure light beam is not led to the cleaning optical system when the photosensitive substrate is irradiated with the exposure light beam. Further, this exposure apparatus may be provided with an alternative light beam source for generating the cleaning light beam, which is disposed separately from the exposure light beam source and emits light beam leading to the cleaning optical system.
The manufacturing method for manufacturing the exposure apparatus according to the present invention is directed to a manufacturing method for manufacturing an apparatus for transferring a pattern formed on a mask onto a substrate, which can achieve the above-mentioned objects by the step of locating an optical system with a part of optical elements arranged so as to be shifted between an operation position and a retraction position in an illumination optical system for illuminating the mask and/or a projection optical system for projecting an image of the pattern on the illuminated mask onto the substrate; and the step of optically cleaning the optical element located in the retraction position by irradiating the optical element with the cleaning light beam.
The exposure method according to the present invention can achieve the above-mentioned objects in an exposure method for projecting an image of a pattern formed on a mask onto a substrate, wherein an exposure light beam is entered into an optical system located between a light beam source emitting the exposure light beam and the substrate by setting an aperture number of the optical system to be larger during optical system is not irradiated with the exposure light beam than an aperture number of the optical system during the exposure and permitting the exposure light beam.
In the exposure method according to the present invention, it is preferred that a light blocking member for blocking the entry of the exposure light beam into the substrate is inserted into a light path located between the optical system and the substrate at the time when no exposure is effected and that the light blocking member is retracted from the light path during the exposure is effected. The light blocking member may comprise a light shielding plate that can be inserted into and retracted from the light path of the exposure light beam, the light shielding plate being interposed, for example, between the optical system and the substrate and disposed in such a way that a side thereof facing the optical system constitutes a reflecting surface. In this case, the reflecting surface of the light shielding plate is preferably set so as to make the angle of a normal of the reflecting plane variable in a predetermined angle range with respect to the optical axis of the optical system. Moreover, it is preferred that the light shielding plate is disposed so as to permit the exposure light beam to enter into the optical system while rocking the light shielding plate. In addition, it is preferred that a second aperture stop is disposed in the optical system, which has an aperture dimension larger at the time of no exposure than that of a first aperture stop for use during the exposure.
An alternative exposure method according to the present invention can achieve the above-mentioned objects in an exposure method for projecting an exposure light beam onto a substrate through a mask, wherein the cleaning light beam is distributed on a pupil plane of an optical system in a wider area than the exposure light beam in order to reduce a variation or fluctuation in the transmittance of the optical system through which the exposure light beam passes.
In this exposure method, it is preferred that the transfer of a pattern formed on the mask onto the substrate is carried out separately from the optical cleaning. Further, it is preferred that an aperture number of the optical system to be determined in accordance with the pattern formed on the mask is set to be larger at the time of the optical cleaning than the aperture number of the optical system at the time of transcribing the pattern onto the substrate. Moreover, it is preferred that the exposure light beam is used as the cleaning light beam and that the entry of the exposure light beam into the substrate is blocked at the time of the optical cleaning of the optical system.
Furthermore, an alternative exposure method according to the present invention can achieve the above-mentioned objects in an exposure method for projecting an image of a pattern formed on a mask onto a substrate through an optical system, wherein a light blocking member for blocking the entry of the exposure light beam into the substrate is inserted into a light path located between the optical system and the substrate during the substrate is not irradiated with the exposure light beam and the light blocking member is retracted from the light path during the substrate is irradiated with the exposure light beam.
In this alternative exposure method, it is preferred that the light blocking member comprises a light shielding plate that is so arranged as to be inserted into the light path for the exposure light beam or retracted therefrom, the light path located between the optical system and the substrate, and that has a plane thereof facing the optical system side arranged so as to act as a reflecting plane. In this case, the reflecting plane of the light shielding plate is set such that the angle of a normal thereof is made variable in a predetermined angle range with respect to the optical axis of the optical system. It is further preferred that the exposure light beam is arranged to enter into the optical system while rocking the light shielding plate.
Moreover, an alternative exposure method according to the present invention can achieve the above-mentioned objects in a projection exposure method for use with a projection exposure apparatus having an illumination optical system for illuminating an exposure light beam emitted from an exposure light beam source onto an original print with a predetermined pattern formed thereon and a projection optical system for projecting the pattern of the original print illuminated by means of the illumination optical system onto a photosensitive substrate, wherein the exposure light beam is emitted from the exposure light beam source so as to enter into the photosensitive substrate when the predetermined pattern formed on the original print is projected onto the photosensitive substrate, and so as to block the entry of the exposure light beam into the photosensitive substrate while allowing the entry of the exposure light beam up to the projection optical system when no pattern of the original print is projected onto the photosensitive substrate.
In the projection exposure method as described above, it is preferred that a light blocking member for blocking the entry of the exposure light beam into the photosensitive substrate is disposed in a position between the projection optical system and the photosensitive substrate. It is further preferred that the light blocking member is controlled so as to allow the entry of the exposure light beam into the photosensitive substrate when the pattern of the original print is projected onto the photosensitive substrate, and so as to block the entry of the exposure light beam into the photosensitive substrate while permitting the entry of the exposure light beam up to the projection optical system when the pattern of the original print is not projected onto the photosensitive substrate.
In addition, an alternative exposure method according to the present invention can achieve the above-mentioned objects in a projection exposure method for use with a projection exposure apparatus comprising an illumination optical system for illuminating an exposure light beam left from an exposure light beam source onto an original print with a predetermined pattern formed thereon, a projection optical system for projecting the pattern of the original print illuminated by means of the illumination optical system onto a photosensitive substrate, and a variable aperture stop disposed inside the projection optical system, wherein the exposure light beam is emitted from the exposure light beam source by opening the variable aperture stop at a predetermined aperture number when the pattern of the original print is projected onto the photosensitive substrate, and the exposure light beam is emitted from the exposure light beam source by fully closing the variable aperture stop when the pattern of the original print is not projected onto the photosensitive substrate.
The exposure apparatus according to the present invention can achieve the above-mentioned objects in a projection exposure apparatus having an illumination optical system for illuminating a mask and a projection optical system for projecting an image of a pattern formed on the mask onto a substrate, wherein a blocking device for blocking the entry of an exposure light beam onto the substrate is provided in a light path located between the projection optical system and the substrate.
In the exposure apparatus according to the present invention, it is preferred that the projection optical system is provided with a variable stop and an aperture number of the variable stop is set so as to become larger at the time when no exposure is effected than at the time when the exposure is being effected. It is further preferred that the blocking device comprises a light shielding plate that can be entered into and retracted from a light path located between the projection optical system and the substrate. Moreover, it is preferred that a side of the light shielding plate faced to the projection optical system can act as a light reflecting surface and further that the angle of a normal of the light reflecting plane thereof is made variable in a predetermined angle range with respect to the optical axis of the optical system. In this case, it is furthermore preferred that a drive unit is installed which can swing the light shielding plate and change the angle of the normal of the light reflecting plane in a predetermined angle range with respect to the optical axis of the optical system. In addition, it is preferred that the variable aperture stop is provided in the illumination optical system and the aperture stop is controlled so as to become larger when the exposure light beam is blocked by means of the light blocking device than when the exposure light beam is exposed.
Further, an alternative exposure apparatus according to the present invention can achieve the above-mentioned objects in a projection exposure apparatus having an illumination optical system for illuminating a mask and a projection optical system for projecting an image of an pattern formed on the mask onto a substrate, wherein a light blocking device for blocking the entry of an exposure light beam into the substrate is disposed in the inside of the projection optical system.
This projection exposure apparatus is preferably provided with a variable aperture stop in the projection optical system, and the variable aperture stop can be preferably arranged so as to act as the light blocking device by fully closing the variable aperture stop. Moreover, it is preferred that a variable aperture stop is disposed in the illumination optical system and the variable aperture stop is controlled so as to become larger when the exposure light beam is blocked by means of the light blocking device than when the exposure is being effected.
Another alternative exposure apparatus according to the present invention is directed to an exposure apparatus for transferring a pattern formed on a mask onto a substrate, and this alternative exposure apparatus can achieve the objects by installing an optical system for projecting an image of the pattern of the mask onto the substrate and a reflecting device having a reflecting surface which reflects an exposure light beam left from the optical system and having a normal of the reflecting surface set so as to be variable in a predetermined angle range with respect to the optical axis of the optical system.
The manufacturing method for manufacturing an exposure apparatus according to the present invention is directed to a manufacturing method for manufacturing an apparatus for transferring a pattern formed on a mask onto a substrate, and this manufacturing method according to the present invention can achieve the above-mentioned objects by the step of locating a light blocking member for blocking the entry of an exposure light beam in a light path located between a projection optical system for projecting the pattern of the mask onto the substrate and the substrate, and the step of optically cleaning the projection optical system by the aid of the entry of the exposure light beam thereinto in such a state that the entry of the exposure light beam into the substrate is blocked by means of the light blocking member.
In this manufacturing method, the light blocking member is controlled so as to block the entry of the exposure light beam into the substrate while permitting the entry of the exposure light beam up to the projection optical system when the pattern is not projected onto the substrate, and so as to allow the entry of the exposure light beam into the substrate when the pattern is projected onto the substrate. It is further preferred that the light blocking member is arranged so as for a side thereof facing the projection optical system to work as a light reflecting surface and that the angle between a normal of the light reflecting surface thereof and the optical axis of projection optical system is variable in a predetermined angle range. Moreover, it is preferred that a drive unit is installed for swinging the light blocking member so as to make the angle between the normal of the light reflecting surface thereof and the optical axis of projection optical system variable in the predetermined angle range.
The optically cleaning method for optically cleaning the exposure apparatus according to the present invention is directed to an optically cleaning method for optically cleaning an exposure apparatus for projecting an image of a pattern formed on a mask onto a substrate through an optical system, and the optically cleaning method according to the present invention can achieve the above-mentioned objects such that the exposure light beam is entered into the optical system by setting an aperture number of an aperture stop for the optical system to be larger when no exposure is conducted than when the exposure is in process.
In this optically cleaning method, the optical system is preferably applicable to an illumination optical system for illuminating the mask with a light beam and/or a projection optical system for projecting an image of a pattern formed on the mask illuminated by means of the illumination optical system onto the substrate. It is preferred that a light blocking member for blocking the entry of the exposure light beam into the substrate is inserted into a light path located between the optical system and the substrate during the optical cleaning. The light blocking member preferably comprises a light shielding plate that can be inserted into and retracted from the light path for the exposure light beam located between the optical system and the substrate, and the light shielding plate is preferably disposed such that a side thereof facing the optical system works as a light reflecting surface. It is further preferred that the angle of a normal of the light reflecting plane thereof is variable in a predetermined angle range with respect to the optical axis of the optical system.
An alternative optically cleaning method according to the present invention can achieve the above-mentioned objects in an optically cleaning method for optically cleaning an exposure apparatus for projecting an image of a pattern formed on a mask onto a substrate through an optical system with an aperture stop installed therein, wherein a cleaning light beam is entered into the projection optical system while fully closing the aperture stop.
Further, another alternative optically cleaning method according to the present invention can achieve the above-mentioned objects in an optically cleaning method for optically cleaning an exposure apparatus for transferring an image of a pattern formed on a mask onto a substrate through a projection optical system, wherein a reflecting member having a light reflecting surface for reflecting an exposure light beam toward the projection optical system side is inserted into a light path interposed between the projection optical system and the substrate and the inside of projection optical system is illuminated with the exposure light beam while varying the angle of the normal line of the light reflecting plane in a predetermined angle range with respect to the optical axis of projection optical system.